balanced preamp

Atmasphere, Fully balanced configuration reduces only even harmonics while leaving odd harmonics intact - I'm not sure if it sounds better. In addition gains have to be perfectly matched or common mode interference would get converted to normal mode signal. It also seriously increases cost - could buy much better non-fully balanced preamp.

Atmasphere, If preamps are balanced and differential then at the spot where summing takes place (input of power amp or speakers) common mode interference will appear as signal. For instance, if one leg has gain of 30 and the other gain of 30.3 than your gains are mismatched 1% meaning your CMRR is limited to poor 40dB and 1% of junk goes thru. We can take for comparison input module in my Rowland 102 with THAT1200 instrumentation amp (instead of transformer) with CMRR=85dB (DC-20kHz).

Many people like warm sound. Removing even harmonics might not to be to their liking. Main beef is with odd harmonics that are left intact (perhaps not big problem in tube amp).

In case of your amplifiers - power supply has to be way heavier while number of tubes and output transformers and expensive capacitors doubles, chassis gets much bigger etc. It should cost a lot more. Your amps are state of the art and might not meet objectives of typical person (most bang for limited bucks).

Atmasphere, 85dB is decent especially when you take into account effect of twisted pair rejection, in addition to shielding, in XLR cable. In order to match 85dB rejection in fully balanced amplifier gains would have to be matched to 0.005% - not possible IMHO.

I forgot, you have no transformers but you have a lot of tubes. Not only cost of tubes but they also require more power.

Atmasphere, We were talking about cost of fully balanced gear. I expressed opinion that in order to get fully balanced amp or preamp you pay more and sacrifice CMRR (since you cannot keep gains equal to 0.005%) - whole thing to remove even harmonics that most people don't mind while leaving odd harmonics intact. I suggested to get better standard amp/preamp instead.

As for 85dB being easy to do - please notice that CMRR of line input transformers is shown at 60Hz. 85dB at 20kHz is pretty good number for any instrumentation amp. In addition line input transformers introduce distortions at low frequencies.

Atmasphere, It seems that fully balanced configuration offers benefit of even harmonics reduction at some expense of CMRR. I can understand that it might be too expensive to manufacture to stay competitive at the lower/medium price range but all your amps are fully balanced (I think).

I often see opinions that XLR cable offers no benefits without fully balanced configuration. Nothing can be further from the truth. Not only that balanced input has common mode noise rejection at the order of 85dB+ (nothing to sneeze at) but also twisted pair (in addition to shielding) has huge rejection especially at audio frequencies (where wavelength is much longer than twist density). In comparison single ended gear has only protection of the shield and, as you stated, zero CMRR.